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Evidence for carbonates on Mars has remained elusive. For example, most remote sensing instruments such as OMEGA and THEMIS that are sensitive to infrared emissivity spectral features of carbonates, have not suggested the presence of carbonate outcrops at 100 m or coarser spatial scales. Though ubiquitous, carbonates dominated by Magnesite (MgCO3) in Martian dust have mass fractions less than 5% and can form under current atmospheric conditions.. Furthermore, with the exception of the surface dust component, carbonates have not been detected by any in situ mission, even though mineralogic modeling does not preclude small amounts of calcium carbonate in Independence class rocks of Husband Hill in Gusev cratermarker (note: An IAU naming convention within Gusev is not yet established).

The first successful identification of a strong infrared spectral signature from surficial carbonate minerals of local scale ( 10 km2) was made by the MRO-CRISM team. Their spectral modeling has identified a key deposit in Nili Fossae dominated by a single mineral phase that is spatially associated with olivine outcrops. The dominant mineral appears to be magnesite, while morphology inferred with HiRISE and thermal properties suggest that the deposit is lithic. Stratigraphically, this layer appears between phyllosilicates below and mafic cap rocks above, temporally between the Noachian and Hesperian eras. Even though infrared spectra are representative of minerals to less than \approx100 micrometre depths (in contrast to gamma spectra which are sensitive to tens of cm depths), stratigraphic, morphologic, and thermal properties are consistent with the existence of the carbonate as outcrop rather than alteration rinds. Nevertheless, the morphology is distinct from typical terrestrial sedimentary carbonate layers suggesting a formation from local aqueous alteration of olivine and other igneous minerals. Key implications are that the alteration would have occurred under moderate pH and that the resulting carbonates were not exposed to sustained low pH aqueous conditions even as recently as the Hesperian. This increases the likelihood of local and regional scale geologic conditions on Mars that were favorable to analogs of terrestrial biological activity over geologically significant intervals.

The absence of more extensive carbonate deposits on Mars may be due to the global dominance of low pH aqueous environments. Even the least soluble carbonate, siderite (FeCO3), precipitates only at a pH greater than 5 The chemical and mineralogic observations by the Mars Exploration Rovers at both Columbia Hills and Meridiani are consistent with low-pH aqueous conditions at the two sites during Noachian and Noachian-Amazonian times, respectively. Sophisticated olivine dissolution models can utilize the presence of olivine bearing outcrops on Mars to predict temporal constraints for such brines (e.g., at a pH of 3.5 and 273 K a 1 mm forsterite grain may survive as long as 140 ka). The temporal resolution will improve once Martian physicochemical conditions are better constrained.

The emerging view of the Martian surface indicates at least three eras of chemical alteration: clays (phyllosian), sulfates (theiikian), and anhydrous ferric oxides (siderikian). If, as postulated by Bibring et al., the Noachian era of clays was spatially widespread with formation in equilbrium with the atmosphere, thermodynamics predicate an early Martian atmosphere that is low in carbon dioxide (partial pressure less than 10³ Pa). Therefore, it may be that the atmosphere of early Mars simply lacked enough CO2 to yield volumetrically significant carbonate deposits. Furthermore, the Hesperian era of sulfates would have been dominated by the low-pH conditions that inhibit carbonate formation as evident in the Columbia Hills of Gusev and the sedimentary outcrop in Meridiani Planum. Alternatively, clays may have formed on early Mars without sustained atmospheric contact, while regional scale - as opposed to local - water bodies may have always been acidic inhibiting the formation of carbonates even in a CO2 rich atmosphere.

Nevertheless, it is intriguing that Shergotty-Nakhla-Chassigny type meteorites from Mars contain evidence for carbonates, albeit at volume fractions less than 1% Furthermore, the enrichment of P - a moderately lithophile element on Mars - and depletion of Si in the Wishstone class float rocks, which potentially dominate the northwest flank of Husband Hill, suggests that they may be associated with carbonatitic magmas. While the bulk of extrusive rocks from corresponding alkaline magmas would contain amounts of carbonates too small to be detected or survive acidic alteration, carbonatite melts in the Martian mantle would contribute to the carbon inventory of bulk Mars. Consistent with the volatile-rich accretion of Mars, such a scenario could imply a greater carbon inventory than the current planetary-mass-adjusted 10−4 to 10−3 estimate relative to that of bulk Earth.

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